The risk of civilians or emergency responders being exposed to ionizing radiation is high in scenarios of nuclear/radiological terrorism or accidents. Reactive oxygen species and electrophiles generated after radiation exposure is involved in impaired immune response due to depletion of hematopoietic immune cells and loss of mucosal barriers, predisposing an individual to secondary infection by opportunistic bacteria and exacerbate radiation-induced morbidity and mortality driven by sepsis. There is an urgent need to develop effective therapies to mitigate synergistic effects of radiation and infection, which can be translated to mass casualty response. Our studies have shown that redox sensitive transcription factor, nuclear factor-erythroid 2 p45-related factor 2 (NRF2) protects against inflammatory disorders caused by environmental oxidants and bacterial infection by inhibiting oxidative stress. Dissociation of the transcription factor from its inhibitor, KEAP1 by electrophiles or oxidants causes nuclear translocation of NRF2 and transcriptional induction of antioxidant genes - glutathione pathway, thioredoxin pathway, heme oxygenase-1, several protective pathways that collectively protect against oxidative stress and macromolecular damage. Our research provides strong rational to hypothesize that "enhancing the Nrf2 pathway will mitigate multi-organ injury by attenuating oxidative stress and improve survival following radiation-combined infection injury". The R21 phase will investigate the strategy of enhancing the Nrf2 pathway to mitigate multi-organ injury and improve survival following radiation-combined bacterial infection injury in mice models by genetic approach (using inducible deletion of Nrf2 inhibitor, KEAP1) and small molecule approach (using a potent Nrf2 activator, CDDO-Me [methyl ester derivative of 2- cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO)] that has undergone phase I clinical trial). Based on a positive outcome of intervening radiation-combined injury using Nrf2 activator, the R33 phase will focus on development of optimal treatment regimen and prognostic biomarkers of Nrf2 based therapy using CDDO-Me in mice model and then perform proof of concept studies in nonhuman primate model focusing on post-radiation induced gastrointestinal injury. These studies promise to develop a novel strategy to mitigate radiation- combined injury that can be used as an effective countermeasure in target population. Page 1 of 1 Public Health Relevance Statement Novel therapy for mitigating and treating radiation as well as radiation combined infection injury is urgently warranted because they present a huge public health problem in radiological/nuclear war /accident scenario. Other than clinical management, currently there are no effective therapies that can be translated to mass causality during a nuclear attack. This application is focused on development of a novel strategy for treating radiation injury by targeting a host defense factor.